Hello Ahmedlmad
When you configure L2 transport on a router’s interface, you enable the interface to carry Layer 2 traffic between two devices directly connected to it, similar to a switch. Essentially, the router behaves as a bridge with the specified interface.
As the encapsulation is not defined, the interface will accept any frame received at the Layer 2 level. This means that the router will forward any received Layer 2 frames across the interface, regardless of their encapsulation type (such as Ethernet, IEEE 802.1Q, or others).
Keep in mind that in order to use such a configured port with features such as MPLS, Metro Ethernet, and others, you may need to configure encapsulation options as well, depending on the specific traffic-handling requirements in each case.
I hope this has been helpful!
Laz